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Water pump and pressure vessel

Does what it says on the tin
genou
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Water pump and pressure vessel

#523260

Postby genou » August 17th, 2022, 8:33 pm

I move hot water to the top floor of the house by using a pump, with a pressure vessel to persuade the pump that it doesn't need to run when there is no demand. The pressure vessel failed while we were away, and the pump cycled for some time before it was switched off. I've replaced the pressure vessel. It all works, but the pump cycles quite frequently under no demand, which is new. What is the next step? I can put more pressure in the vessel, but is that a potential solution ?

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Re: Water pump and pressure vessel

#523295

Postby servodude » August 17th, 2022, 11:15 pm

genou wrote:I move hot water to the top floor of the house by using a pump, with a pressure vessel to persuade the pump that it doesn't need to run when there is no demand. The pressure vessel failed while we were away, and the pump cycled for some time before it was switched off. I've replaced the pressure vessel. It all works, but the pump cycles quite frequently under no demand, which is new. What is the next step? I can put more pressure in the vessel, but is that a potential solution ?


I think it might be.

Sounds like the system just needs a bit of tuning (shifting the activation point); that could be adjusting the pressure to bias it or adjusting however you convey that info to the pump (I've not really got a picture of how you're doing it in my head)

-sd

Itsallaguess
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Re: Water pump and pressure vessel

#523315

Postby Itsallaguess » August 18th, 2022, 6:10 am

genou wrote:
I move hot water to the top floor of the house by using a pump, with a pressure vessel to persuade the pump that it doesn't need to run when there is no demand.

The pressure vessel failed while we were away, and the pump cycled for some time before it was switched off.

I've replaced the pressure vessel.

It all works, but the pump cycles quite frequently under no demand, which is new.

What is the next step?


I can't think of a system like that which can work without also incorporating a pressure switch to both send demand to the pump when it senses a need for hot-water pressure, and then turns the pump back off once that demand stops.

If such a pressure switch isn't an integral part of the existing pump, then given that you've not mentioned a pressure switch as part of your investigation, and it sounds like you're still having similar pump-cycling problems either side of the recent pressure-vessel replacement, then I'd probably turn my attention to finding if there is a pressure switch, and perhaps focussing on that if there is one...

Cheers,

Itsallaguess

Itsallaguess
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Re: Water pump and pressure vessel

#523317

Postby Itsallaguess » August 18th, 2022, 6:57 am

genou wrote:
It all works, but the pump cycles quite frequently under no demand, which is new.


As well as a possible pressure-switch investigation as mentioned earlier, it will be worth checking to see if there's a relief-valve anywhere on the hot-water circuit, because if you've had pump over-run issues then there's a chance the relief-valve might have been activated, and if that's the case then it's not an uncommon situation for such relief valves to subsequently 'let-through' periodically, which might then also help to explain your current pump-cycling issues...

Usually such relief valves go to an exposed bit of open pipe outside of the building, and elastic-banding a clear plastic bag over the open end of the outside pipe can often show any potentially relevant water finding it's way past the valve...

Separately, and just for clarity - can we assume that we're talking about domestic hot-water here, out of the upstairs taps, or are we talking about a specific hot-water feed to a shower, or is this central-heating hot-water for radiators we're talking about here? I'm very concious that it's not been specified yet, and I think it's going to be relevant to better understand just what this upstairs hot-water is being used for?

Cheers,

Itsallaguess

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Re: Water pump and pressure vessel

#523404

Postby 88V8 » August 18th, 2022, 10:15 am

genou wrote:I move hot water to the top floor of the house by using a pump, with a pressure vessel...

Is/was the pressure vessel red, or blue?

V8

genou
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Re: Water pump and pressure vessel

#523526

Postby genou » August 18th, 2022, 4:43 pm

Thanks to all for the assistance. This is rather long, but I hope it addresses all the questions asked.

I am not conscious that there is a pressure relief valve anywhere.

The house is 3 stories. The original design had a hot tank and header on the first floor, with no water of any kind to the top floor. The current ( but now 10+ years old ) setup moves cold under mains pressure, and hot by the pumping we are discussing to the top floor to supply a loo, a handbasin and a mixer shower. It worked quite happily for several years until the diaphragm in the old pv failed, and it does still work , subject to the fact that the pump is cycling more than I'd like. It only runs for a second until it has raised the pressure, and then it cuts out; but it will do it several times a night at current rates.

The pressure vessel is / was stainless steel - it is now a Zilmet Inox-Pro Stainless Steel Expansion Vessel - 10bar - 0.5Ltr
( https://www.pumpsalesdirect.co.uk/zilme ... 10bar.html )

This looks to me to be a direct replacement for the failed unit ( it's not identical - they seem to have moved from 3/4" to 1/2" connections, but I fixed that with a reducer ).

The pump is labelled Monsoon Extra N1.3bar Single, and close relatives seem to be still in production. I'd always imagined that the pressure switch was in the pump, and the purpose of the pressure vessel was to persuade the pump that there was no demand, by compensating for whatever loss of pressure occurs by [ waves hands having no proper idea why there should be a loss of pressure ]

This is probably the crux - I've never grasped why the pressure should fall, and put it down to the pump just gently backfilling into it's own feed from the hot tank where the pressure is merely a header tank a few feet above, and therefore less than the pressure the pump has created in the supply pipework above it. Hence my thought that if I give it more pressure to lose, it will take longer to lose it.

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Re: Water pump and pressure vessel

#523540

Postby MyNameIsUrl » August 18th, 2022, 5:34 pm

genou wrote:This is probably the crux - I've never grasped why the pressure should fall, and put it down to the pump just gently backfilling into it's own feed from the hot tank where the pressure is merely a header tank a few feet above, and therefore less than the pressure the pump has created in the supply pipework above it. Hence my thought that if I give it more pressure to lose, it will take longer to lose it.

A stationary pump impeller will allow flow (in either direction) if there is a pressure differential across it. Have you thought of inserting a non-return valve to see if that helps?

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Re: Water pump and pressure vessel

#523557

Postby Itsallaguess » August 18th, 2022, 6:29 pm

genou wrote:
The pressure vessel is / was stainless steel - it is now a Zilmet Inox-Pro Stainless Steel Expansion Vessel - 10bar - 0.5Ltr

( https://www.pumpsalesdirect.co.uk/zilme ... 10bar.html )


These pressure vessels are in hot-water legs like this to take up the additional expansion of hot-water as it warms up in the system, and when things 'run hot' for a while, and are then shut off, and then the shower pump stops running as well, the water in that initially 'hot leg' of pipework will subsequently and slowly cool down, and the pressure vessel diaphragm will then expand back to maintain some of that 'previously hot water' pressure as the remaining water cools in that section of the system, and so if the initial charging of the pressure vessel has not been carried out correctly, to enable the correct operating pressure of the system to be properly achieved to start with, there's a chance that when the hot-water pipework leg subsequently cools down currently, there isn't enough pressure being put back onto the water-side by the vessel, thus making the pump think there's a small demand occurring when that cooling-down pressure goes too low...

Might the above explanation correspond in terms of the timing of when you've recently noticed the pump kicking in when you'd not expect it to?

Is it during a period where previously hot water in that section of the pipework might then be cooling down from recently being used?

Cheers,

Itsallaguess
Last edited by Itsallaguess on August 18th, 2022, 6:35 pm, edited 1 time in total.

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Re: Water pump and pressure vessel

#523559

Postby BullDog » August 18th, 2022, 6:34 pm

Dead simple -

Instead of using a pressure switch, change it to a flow switch to start the pump. When you turn a tap on the flow switch will start the pump. Turn the tap off and the flow/pump will stop.

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Re: Water pump and pressure vessel

#523561

Postby BullDog » August 18th, 2022, 6:41 pm

BullDog wrote:Dead simple -

Instead of using a pressure switch, change it to a flow switch to start the pump. When you turn a tap on the flow switch will start the pump. Turn the tap off and the flow/pump will stop.

Something like this.......

https://mac3uk.com/product/home-boostin ... ch-e-flow/

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Re: Water pump and pressure vessel

#523598

Postby genou » August 18th, 2022, 8:38 pm

I'll have a play and report back. I doubt it is cooling, because surely that is a one off. I'm not minded to introduce new switching, although I see why, because I'd rather move to mains pressure hot water. But that's not for today - marking time until I get a proper heat pump solution.

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Re: Water pump and pressure vessel

#523599

Postby BullDog » August 18th, 2022, 8:55 pm

genou wrote:I'll have a play and report back. I doubt it is cooling, because surely that is a one off. I'm not minded to introduce new switching, although I see why, because I'd rather move to mains pressure hot water. But that's not for today - marking time until I get a proper heat pump solution.

What you have discovered is that a fluid filled pressurised volume has the big disadvantage that a very tiny loss of fluid means a significant loss of pressure. It's physics. So the pump cycles. It's really a very bad idea you have. It will never be as reliable as a flow switch. Ofcourse, it's entirely up to you how you address the problem. But fundamentally, it's unreliable what you're doing with a pressure switch. That's why flow switches exist.

Good luck anyway whatever you decide to do.

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Re: Water pump and pressure vessel

#523624

Postby 9873210 » August 19th, 2022, 12:38 am

While in general I'd prefer a flow switch, ...

One thing that hasn't been mentioned is set points and hysteresis on the pressure switch. If you are going to use a pressure switch and an accumulator the pump should turn on when the accumulator is almost empty and turn off when the accumulator is almost full. A good pressure switch should have adjustments for these two set points.

The gas charge in the accumulator can also affect this and should be matched to the switch pressure set points (or vice versa). Too little or too much gas charge is equally bad. For an undercharge the pump will turn on while the accumulator is still mostly full. For an overcharge the pump will turn off while the accumulator is still mostly empty. Either case restricts the working range of the accumulator. So it's possible you need to let air out of the accumulator.

Edit to add:
One test you might be able to do is to run water, close the tap, wait until the pump shuts off and then crack the tap and measure how much water comes out before the pump kicks back on. If it's over 0.25L you're using over half the capacity of the tank. If it's much less adjustments may be helpful.

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Re: Water pump and pressure vessel

#523642

Postby Itsallaguess » August 19th, 2022, 7:13 am

genou wrote:
I'll have a play and report back.

I doubt it is cooling, because surely that is a one off.


If it's the subsequent water cooling that's cycling the pump via a drop in water-pressure, then it's curve-based rather than a 'one off' process, so it might cool a bit, drop pressure a bit, and eventually momentarily activate the pump via it's own pressure switch, which would then re-pressurise the 'still cooling' water to a point where the pump is not being called for, but where the underlying water is perhaps then still cooling, and so that pump-cycling might occur a number of times until the water loses enough temperature (and hence pressure...) to settle down into a final 'cool but not cooling down any more' state...

Cheers,

Itsallaguess


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